Certain substances (e.g., salt, etc.) have low permeability, i.e., do not readily transmit fluids. In some cases, a low permeability substance may be located above a substance that is targeted by a drilling operation.
FIG. 1 shows an exemplary diagram of a drilling operation. Those skilled in the art will appreciate that the drilling operation shown in FIG. 1 is provided for exemplary purposes only and accordingly should not be construed as limiting the scope of the invention. For example, the drilling operation shown in FIG. 1 is a seafloor drilling operation, but the drilling operation may alternatively be a land drilling operation.
As shown in FIG. 1, a drilling rig (105) is configured to drill into a formation (e.g., a formation below a seafloor (110)) using a drill bit (not shown) coupled to the distal end of a drill string (125). Specifically, the drill bit is used to drill a borehole (130) extending to a target substance (120). The target substance (120) may be hydrocarbon or a mineral resource targeted by a drilling operation. Further, as shown in FIG. 1, a low permeability substance (115) may be located above the target substance (120). Thus, to reach the target substance (120), the borehole (130) may be required to pass through the low permeability substance (115).
When sediment of low permeability substance is buried or compacted, fluid may be trapped in pores within the resulting structure (i.e., within the low permeability substance itself and/or within substances beneath the low permeability substance (e.g., sand, etc.). Fluid trapped in this manner exerts pressure on the surrounding formation referred to as pore pressure. Formations in which pore pressure exceeds hydrostatic pressure at a given depth are referred to as overpressured.
When drilling in an overpressured formation, the mud weight (i.e., the weight of drilling fluids transmitted to the borehole) must be high enough to prevent the pore pressure from moving formation fluids into the borehole. In the worst case, formation fluids entering a borehole may result in loss of the well and/or injury to personnel operating the drilling rig. Accordingly, for safe and economic drilling, it is essential that the pore pressure be predicted with sufficient accuracy. In particular, it is beneficial to predict pore pressure pre-drill, i.e., either before any drilling has commenced and/or at a location that the drill bit has not yet reached.
Conventionally, pre-drill pore pressure prediction is based on the use of pre-drill seismic velocities and a velocity-to-pore pressure transform calibrated using offset well data (i.e., data from other wells near the drilling site). However, in some cases (e.g., when drilling under salt), conventional pre-drill pore pressure predictions may not be sufficiently accurate. Further discussion of conventional pre-drill pore pressure prediction techniques can be found in Sayers C M, Johnson G M, and Denyer G., 2002, “Pre-drill Pore Pressure Prediction Using Seismic Data,” Geophysics, 67, 1286-1292.
Mud is used in drilling operations to cool the drill bit, to transport cuttings generated by the drilling operation to the surface, to prevent the influx of formation fluids into the borehole, and to stabilize the borehole. With respect to preventing the influx of formation fluids, the drilling operator must maintain the mud weight at or above the pore pressure. With respect to stabilizing the borehole, drilling operators adjust the mud weight (i.e., the density of the mud being used) to counter the tendency of the borehole to cave in. However, the drilling operator must be careful not to fracture the formation by using an excessively high mud weight.
Moreover, too high a mud weight may result in an unacceptably low drilling rate. Accordingly, the mud weight must be low enough to maintain an acceptable drilling rate and avoid fracturing the formation. In such cases, the allowable mud weight window (i.e., the range of allowable mud weights) may be small when drilling in overpressured formations. Specifically, the force exerted by the mud must fall within the range between the pore pressure (or the pressure to prevent a cave in, if higher than the pore pressure) and the pressure required to fracture the formation.
Further, when drilling in overpressured formations, the number of required casing strings (i.e., structural supports inserted into the borehole) may be increased. Specifically, if a sufficiently accurate pre-drill pore pressure prediction is not available, additional casing strings may be inserted prematurely, to avoid the possibility of well control problems (e.g., influx of formation fluids) and/or borehole failure. Prematurely inserting casing strings may delay the drilling operation and/or reduce the size of the borehole and result in financial loss.